Frequently Asked Questions

Below is a list of frequently asked questions related to flood control and stormwater management. Have a question you’d like added to this list? Super — Please use the form below to enter your query and we’ll do our best to answer and post it to this page.


Q: Why is it called a ‘100 Year Flood’ if it happens every year?

A 100 Year Flood is the 1% chance of flooding in any given year –

Q: What is the ‘500 Year Flood’?

A 500 Year Flood is the 0.2% chance of flooding in any given year –

Q:  What is the textbook definition of a flood?

A flood is generally defined as an overflow of water onto land (that is dry most of the time) or into human structures such that damage occurs. More formally, the Federal Emergency Management Agency (FEMA) defines a flood as, “A general and temporary condition of partial or complete inundation of two or more acres of normally dry land area or of two or more properties” (FEMA, NFIP).

For the purpose of this study, flooding is divided into two broad categories. One is flooding on a floodplain (“stream flooding;” also known as overbank or riverine flooding) and the other involves flooding outside of mapped floodplains (“stormwater flooding;” also known as localized flooding, drainage flooding, or overland flow.)

Q: What is river/stream flooding?

River or stream flooding results when the water level in the stream channel rises above its banks. This may be caused by excessive rain or snowmelt, or when the channel is blocked by ice or debris. In either case, water overflows the channel onto surrounding floodplain areas. Such high-risk areas are classified by FEMA as Special Flood Hazard Areas (SFHAs), with the goal of discouraging new construction in these areas and encouraging protection, mitigation measures, and flood insurance coverage for existing structures on US floodplains.

What is stormwater flooding?

Not all flooding occurs on floodplains. Many other locations may experience standing water and damage if the accumulation of water, typically after heavy rains, exceeds the rate at which water drains away from the land. Runoff water collects in low-lying areas until it drains out, infiltrates into the soil, evaporates, or is pumped to another location. This type of flooding can be especially problematic in urban areas where rooftops and pavement increase the amount of runoff after storms.

What are the potential contaminants that are contained in stormwater?
Stormwater can contain varying levels of several potential contaminants, such as sediment and suspended solids, agricultural products like phosphorus fertilizers or pesticides, road oil and grease, road salt, and bacteria. These compounds are picked up by stormwater as it runs across lawns, pavements, and other surfaces where they are present, or occasionally in the case of bacteria, via illegal cross connections from the sanitary sewer system.

Is there any way to deal with pollution once it enters a body of water?
Certain stormwater pollutants, such as discreet spills or accumulations of litter, can be mitigated after reaching a body of water, but other compounds that are highly water-soluble – like road salt residue or phosphorus – are much more difficult to address once they reach a waterway. That is why all effective and state-of-the-art water quality programs attempt to address water pollution as close to the source as possible. This is also why the Village is planning on addressing stormwater quality at locations throughout the system, in addition to the pipe discharge point


Why can’t green infrastructure remedy all flooding problems?
Green infrastructure delivers results, but not on a scale that can help alleviate structural flooding. Green infrastructure does handle smaller storms well and plays a critical role in helping to protect water quality. However, for very large storms, there is no replacement for traditional stormwater management. A simple calculation of how much green infrastructure would be needed to absorb 1 inch of rainfall over 900 acres helps to demonstrate why:

  • One inch rainfall on 900 acres= 75 acre-feet (24,437,160 gallons)
  • Pervious Pavement:
  • One mile of pervious pavement (24 ft. wide with 2 ft. stone base) will store 2 acre-feet.
  • Reconstructing every street between Willow and Pine west of Locust to Hibbard yields 4.8 miles of pervious pavement, storing 9.6 acre-feet of water.
  • 9.6 acre-feet is equivalent to 0.13 inches of rainfall over the watershed. Rain gardens and bio-swales are other forms of green infrastructure that absorb more water than lawns. So what if every property owner in the 900 acre drainage area installed a rain garden amounting to 5 percent of their lot area? For an 8,000 sq. ft. lot, this would amount to replacing 400 sq. ft. of lawn with rain garden. Engineers modeled the increased permeability and found that over 900 acres, runoff would be reduced by 3.7 acre feet. 3.7 acre feet over 900 acres translates to capturing 0.05 inches of precipitation.


Total 100-year runoff storage volume required from 900 acres:

  • 167 acre-feet
  • Less 4.8 miles of pervious pavement (-9.6 acre feet)
  • Less 5 percent of every lot replaced with rain garden (-3.7 acre feet)
  • Less 3, 55-gallon rain barrels per property @ 1200 properties (-0.6 acre-feet)
  • Remaining runoff to be managed = 153.1 acre-feet, or 92 percent of volume.

Green infrastructure certainly has a role to play, and the Village encourages its use where and when it can deliver results. However, given the significant challenges Winnetka faces in managing stormwater more effectively, it is only part of the solution.


Winnetka Town Hall Questions & Answers


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